Controlled Deposition of Silica on Titania-Silica to Alter the Active Site Surroundings on Epoxidation Catalysts
M. Alexander Ardagh, Daniel T. Bregante, David W. Flaherty, Justin M. Notestein
Abstract
Lewis acidic Ti-SiO2 materials are workhorse oxidation catalysts, and they range from microporous substituted zeolites to meso/macroporous materials, often with tradeoffs between steric accessibility and activity. In this study, SiO2 is deposited over the active sites of a macroporous, highly dispersed Ti-SiO2 catalyst, with or without an organic template. SiO2 deposition is shown to impact the local environment around epoxidation active sites without altering the active site or introducing diffusion limitations for the representative bulky alkene limonene. Thus, intrinsic activation enthalpies remain nearly constant (45 ± 3 kJ/mol) across all materials and controls, but templated SiO2 deposition gives an apparent activation enthalpy (9 kJ/mol) much lower than that of the other materials tested because of its strong limonene adsorption. This demonstrates independent control of the active site, its immediate surroundings, and the extended pore structure, while also serving the practical purpose of creating a material that can outperform relevant benchmark materials. These types of materials may find utility in the selective transformation of larger reactants, including biorenewables such as limonene, or precursors for pharmaceuticals and other fine chemicals.